1. Field of the Invention
The present invention relates to a linear actuator obtaining a linear thrust force by electromagnetic force and an apparatus utilizing the linear actuator.
2. Description of the Related Art
One of types of linear actuators is constructed so as to dispose an armature coil in an electric field generated by a permanent magnet. Electric current is supplied to the armature coil so that linear thrust is obtained by electromagnetic force. FIG. 13 illustrates a conventional linear actuator of the above-described type. This actuator is disclosed by JP-A-2004-88992. The convention linear actuator comprises a cylindrical inner and outer yokes 101 and 102. The inner yoke 101 is inserted inside the outer yoke 102 so as to be axially movable. The inner yoke 101 has an outer periphery to which three cylindrical permanent magnets 103 are axially joined. Each permanent magnet 103 includes an inner peripheral portion which is magnetized so as to have either south pole or north pole. Each permanent magnet 103 further includes an outer peripheral portion which is magnetized so as to have either north pole or south pole. The permanent magnets 103 are arranged in an axial sequence of north pole, south pole and north pole in the inner peripheral portions of the magnets and in an axial sequence of south pole, north pole and south pole in the outer peripheral portions of the magnets. Three cylindrical armature coils 104 are inserted in a space defined between the outer peripheral surfaces of the permanent magnets 103 and an inner peripheral surface of the outer yoke 102. The armature coils 104 are mechanically interconnected by the outer yoke 102. Each armature coil 104 is placed in magnetic field. Electric current is applied to each armature coil 104 so that downward or upward thrust is imparted to the outer yoke 102. The permanent magnets 103 and the armature coils 104 are arranged in three stages in the above-described conventional linear actuator. As a result, an axial dimension or the height of the actuator is increased. Moreover, since the axially adjacent permanent magnets 103 are in mechanical contact with each other, magnetic flux loops directly between the axially adjacent permanent magnets 103. Accordingly, an amount of magnetic flux interlinking the armature coils 104 is reduced such that the thrust of the actuator is also reduced.